26 Answers
26

The last two are identical; "atomic" is the default behavior (note that it is not actually a keyword; it is specified only by the absence of nonatomic -- atomic was added as a keyword in recent versions of llvm/clang).

Assuming that you are @synthesizing the method implementations, atomic vs. non-atomic changes the generated code. If you are writing your own setter/getters, atomic/nonatomic/retain/assign/copy are merely advisory. (Note: @synthesize is now the default behavior in recent versions of LLVM. There is also no need to declare instance variables; they will be synthesized automatically, too, and will have an _ prepended to their name to prevent accidental direct access).

With "atomic", the synthesized setter/getter will ensure that a whole value is always returned from the getter or set by the setter, regardless of setter activity on any other thread. That is, if thread A is in the middle of the getter while thread B calls the setter, an actual viable value -- an autoreleased object, most likely -- will be returned to the caller in A.

In nonatomic, no such guarantees are made. Thus, nonatomic is considerably faster than "atomic".

What "atomic" does not do is make any guarantees about thread safety. If thread A is calling the getter simultaneously with thread B and C calling the setter with different values, thread A may get any one of the three values returned -- the one prior to any setters being called or either of the values passed into the setters in B and C. Likewise, the object may end up with the value from B or C, no way to tell.

Ensuring data integrity -- one of the primary challenges of multi-threaded programming -- is achieved by other means.

Adding to this:

atomicity of a single property also cannot guarantee thread safety when multiple dependent properties are in play.

In this case, thread A could be renaming the object by calling setFirstName: and then calling setLastName:. In the meantime, thread B may call fullName in between thread A's two calls and will receive the new first name coupled with the old last name.

To address this, you need a transactional model. I.e. some other kind of synchronization and/or exclusion that allows one to exclude access to fullName while the dependent properties are being updated.

Given that any thread-safe code will be doing its own locking etc, when would you want to use atomic property accessors? I'm having trouble thinking of a good example.
– Daniel DickisonMay 24 '11 at 20:00

8

@bbum Makes sense. I like your comment to another answer that thread-safety is more a model-level concern. From an IBM thread safety definition: ibm.co/yTEbjY "If a class is correctly implemented, which is another way of saying that it conforms to its specification, no sequence of operations (reads or writes of public fields and calls to public methods) on objects of that class should be able to put the object into an invalid state, observe the object to be in an invalid state, or violate any of the class's invariants, preconditions, or postconditions."
– Ben FlynnJan 26 '12 at 19:22

6

Here's an example similar to @StevenKramer 's: I have a @property NSArray* astronomicalEvents; that lists data I want to display in the UI. When the application launches the pointer points to an empty array, then the app pulls data from the web. When the web request completes (in a different thread) the app builds a new array then atomically sets the property to a new pointer value. It's thread safe and I didn't have to write any locking code, unless I'm missing something. Seems pretty useful to me.
– bugloafFeb 15 '13 at 15:52

9

@HotLicks Another fun one; on certain architectures (Can't remember which one), 64 bit values passed as an argument might be passed half in a register and half on the stack. atomic prevents cross-thread half-value reads. (That was a fun bug to track down.)
– bbumNov 23 '13 at 23:19

This is explained in Apple's documentation, but below are some examples of what is actually happening. Note that there is no "atomic" keyword, if you do not specify "nonatomic" then the property is atomic, but specifying "atomic" explicitly will result in an error.

Basically, the atomic version has to take a lock in order to guarantee thread safety, and also is bumping the ref count on the object (and the autorelease count to balance it) so that the object is guaranteed to exist for the caller, otherwise there is a potential race condition if another thread is setting the value, causing the ref count to drop to 0.

There are actually a large number of different variants of how these things work depending on whether the properties are scalar values or objects, and how retain, copy, readonly, nonatomic, etc interact. In general the property synthesizers just know how to do the "right thing" for all combinations.

@fyolnish I'm not sure what _val/val are, but no, not really. The getter for an atomic copy/retain property needs to ensure that it does not return an object whose refcount becomes zero due the setter being called in another thread, which essentially means it needs to read the ivar, retain it while ensuring that the setter hasn't overwritten-and-released it, and then autorelease it to balance the retain. That essentially means both the getter and setter have to use a lock (if the memory layout was fixed it should be doable with CAS2 instructions; alas -retain is a method call).
– tc.Dec 10 '13 at 11:09

@tc It's been quite a while but what I meant to write was probably this: gist.github.com/fjolnir/5d96b3272c6255f6baae But yes it is possible for the old value to be read by a reader before setFoo: returns, and released before the reader returns it. But maybe if the setter used -autorelease instead of -release, that would fix that.
– FjölnirDec 11 '13 at 5:47

The best way to understand the difference is using the following example.

Suppose there is an atomic string property called "name", and if you call [self setName:@"A"] from thread A, call [self setName:@"B"] from thread B, and call [self name] from thread C, then all operations on different threads will be performed serially which means if one thread is executing a setter or getter, then other threads will wait.

This makes property "name" read/write safe, but if another thread, D, calls [name release] simultaneously then this operation might produce a crash because there is no setter/getter call involved here. Which means an object is read/write safe (ATOMIC), but not thread-safe as another threads can simultaneously send any type of messages to the object. The developer should ensure thread-safety for such objects.

If the property "name" was nonatomic, then all threads in above example - A,B, C and D will execute simultaneously producing any unpredictable result. In case of atomic, either one of A, B or C will execute first, but D can still execute in parallel.

The syntax and semantics are already well-defined by other excellent answers to this question. Because execution and performance are not detailed well, I will add my answer.

What is the functional difference between these 3?

I'd always considered atomic as a default quite curious. At the abstraction level we work at, using atomic properties for a class as a vehicle to achieve 100% thread-safety is a corner case. For truly correct multithreaded programs, intervention by the programmer is almost certainly a requirement. Meanwhile, performance characteristics and execution have not yet been detailed in depth. Having written some heavily multithreaded programs over the years, I had been declaring my properties as nonatomic the entire time because atomic was not sensible for any purpose. During discussion of the details of atomic and nonatomic properties this question, I did some profiling encountered some curious results.

Execution

Ok. The first thing I would like to clear up is that the locking implementation is implementation-defined and abstracted. Louis uses @synchronized(self) in his example -- I have seen this as a common source of confusion. The implementation does not actually use @synchronized(self); it uses object level spin locks. Louis's illustration is good for a high-level illustration using constructs we are all familiar with, but it's important to know it does not use @synchronized(self).

Another difference is that atomic properties will retain/release cycle your objects within the getter.

Performance

Here's the interesting part: Performance using atomic property accesses in uncontested (e.g. single-threaded) cases can be really very fast in some cases. In less than ideal cases, use of atomic accesses can cost more than 20 times the overhead of nonatomic. While the Contested case using 7 threads was 44 times slower for the three-byte struct (2.2 GHz Core i7 Quad Core, x86_64). The three-byte struct is an example of a very slow property.

Interesting side note: User-defined accessors of the three-byte struct were 52 times faster than the synthesized atomic accessors; or 84% the speed of synthesized nonatomic accessors.

Objects in contested cases can also exceed 50 times.

Due to the number of optimizations and variations in implementations, it's quite difficult to measure real-world impacts in these contexts. You might often hear something like "Trust it, unless you profile and find it is a problem". Due to the abstraction level, it's actually quite difficult to measure actual impact. Gleaning actual costs from profiles can be very time consuming, and due to abstractions, quite inaccurate. As well, ARC vs MRC can make a big difference.

So let's step back, not focussing on the implementation of property accesses, we'll include the usual suspects like objc_msgSend, and examine some real-world high-level results for many calls to a NSString getter in uncontested cases (values in seconds):

As you have probably guessed, reference count activity/cycling is a significant contributor with atomics and under ARC. You would also see greater differences in contested cases.

Although I pay close attention to performance, I still say Semantics First!. Meanwhile, performance is a low priority for many projects. However, knowing execution details and costs of technologies you use certainly doesn't hurt. You should use the right technology for your needs, purposes, and abilities. Hopefully this will save you a few hours of comparisons, and help you make a better informed decision when designing your programs.

So if atomic properties are bad y are they default. To increase the boilerplate code ?
– Kunal BalaniSep 17 '13 at 20:44

@LearnCocos2D i just tested on 10.8.5 on the same machine, targeting 10.8, for the single threaded uncontested case with an NSString which is not immortal: -ARC atomic (BASELINE): 100% -ARC nonatomic, synthesised: 94% -ARC nonatomic, user defined: 86% -MRC nonatomic, user defined: 5% -MRC nonatomic, synthesised: 19% -MRC atomic: 102% -- the results are a little different today. I wasn't doing any @synchronized comparisons. @synchronized is semantically different, and I don't consider it a good tool if you have nontrivial concurrent programs. if you need speed, avoid @synchronized.
– justinSep 23 '13 at 13:34

Thread safety:

Instance variables are thread-safe if they behave correctly when accessed from multiple threads, regardless of the scheduling or interleaving of the execution of those threads by the runtime environment, and with no additional synchronization or other coordination on the part of the calling code.

In our context:

If a thread changes the value of the instance the changed value is available to all the threads, and only one thread can change the value at a time.

Where to use atomic:

if the instance variable is gonna be accessed in a multithreaded environment.

Implication of atomic:

Not as fast as nonatomic because nonatomic doesn't require any watchdog work on that from runtime .

Where to use nonatomic:

If the instance variable is not gonna be changed by multiple threads you can use it. It improves the performance.

Everything you say here is correct, but the last sentence is essentially "wrong", Dura, for today's programming. It's really inconceivable you would bother to try to "improve performance" this way. (I mean, before you got within lightyears of that, you would be "not using ARC", "not using NSString because it is slow!" and so on.) To make an extreme example, it would be like saying "team, don't put any comments in the code, as it slows us down." There is no realistic development pipeline where you would want the (nonexistent) theoretical performance gains at the sake of unreliability.
– FattieJun 1 '14 at 8:37

I found a pretty well put explanation of atomic and non-atomic properties here. Here's some relevant text from the same:

'atomic' means it cannot be broken down.
In OS/programming terms an atomic function call is one that cannot be interrupted - the entire function must be executed, and not swapped out of the CPU by the OS's usual context switching until it's complete. Just in case you didn't know: since the CPU can only do one thing at a time, the OS rotates access to the CPU to all running processes in little time-slices, to give the illusion of multitasking. The CPU scheduler can (and does) interrupt a process at any point in its execution - even in mid function call. So for actions like updating shared counter variables where two processes could try to update the variable at the same time, they must be executed 'atomically', i.e., each update action has to finish in its entirety before any other process can be swapped onto the CPU.

So I'd be guessing that atomic in this case means the attribute reader methods cannot be interrupted - in effect meaning that the variable(s) being read by the method cannot change their value half way through because some other thread/call/function gets swapped onto the CPU.

Because the atomic variables can not be interrupted, the value contained by them at any point is (thread-lock) guaranteed to be uncorrupted, although, ensuring this thread lock makes access to them slower. non-atomic variables, on the other hand, make no such guarantee but do offer the luxury of quicker access. To sum it up, go with non-atomic when you know your variables won't be accessed by multiple threads simultaneously and speed things up.

Atomic accessors in a non garbage collected environment (i.e. when using retain/release/autorelease) will use a lock to ensure that another thread doesn't interfere with the correct setting/getting of the value.

It is not actually a keyword.

Example:

@property (retain) NSString *name;
@synthesize name;

nonatomic

nonatomic means multiple thread access the variable (dynamic type).

nonatomic is thread-unsafe.

But it is fast in performance

nonatomic is NOT default behavior. We need to add the nonatomic keyword in the property attribute.

It may result in unexpected behavior, when two different process (threads) access the same variable at the same time.

Easiest answer first: There's no difference between your second two examples. By default, property accessors are atomic.

Atomic accessors in a non garbage collected environment (i.e. when using retain/release/autorelease) will use a lock to ensure that another thread doesn't interfere with the correct setting/getting of the value.

See the "Performance and Threading" section of Apple's Objective-C 2.0 documentation for some more information and for other considerations when creating multi-threaded apps.

Two reasons. First off, for synthesized code it generates faster (but not threadsafe code). Second, if you are writing customer accessors that are not atomic it lets you annotate for any future user that the code is not atomic when they are reading its interface, without making them implementation.
– Louis GerbargFeb 26 '09 at 6:34

Atomic :

Atomic guarantees that access to the property will be performed in an atomic manner. E.g. it always return a fully initialised objects, any get/set of a property on one thread must complete before another can access it.

If you imagine the following function occurring on two threads at once you can see why the results would not be pretty.

-(void) setName:(NSString*)string
{
if (name)
{
[name release];
// what happens if the second thread jumps in now !?
// name may be deleted, but our 'name' variable is still set!
name = nil;
}
...
}

Pros :
Return of fully initialised objects each time makes it best choice in case of multi-threading.

That comment doesn't make a lot of sense. Can you clarify? If you look at examples on the Apple site then the atomic keyword synchronizes on the object while updating its properties.
– Andrew GrantFeb 26 '09 at 7:31

Atomic is thread safe, it is slow and it well-assures (not guaranteed) that only the locked value is provided no matter how many threads are attempting access over the same zone. When using atomic, a piece of code written inside this function becomes the part of the critical section, to which only one thread can execute at a time.

It only assures the thread safety; it does not guarantee that. What I mean is you hire an expert driver for you car, still it doesn't guarantees car won't meet an accident. However, probability remains the slightest.

Atomic - it can't be broken down, so the result is expected. With nonatomic - when another thread access the memory zone it can modify it, so the result is unexpected.

Code Talk :

Atomic make getter and setter of the property thread safe. for example if u have written :

The default is atomic, this means it does cost you performance whenever you use the property, but it is thread safe. What Objective-C does, is set a lock, so only the actual thread may access the variable, as long as the setter/getter is executed.

On the other hand does nonatomic add nothing to your code. So it is only thread safe if you code security mechanism yourself.

@property(nonatomic, retain) UITextField *userName;

The keywords doesn't have to be written as first property attribute at all.

Don't forget, this doesn't mean that the property as a whole is thread-safe. Only the method call of the setter/getter is. But if you use a setter and after that a getter at the same time with 2 different threads, it could be broken too!

Atomic is the default: if you don’t type anything, your property is
atomic. An atomic property is guaranteed that if you try to read from
it, you will get back a valid value. It does not make any guarantees
about what that value might be, but you will get back good data, not
just junk memory. What this allows you to do is if you have multiple
threads or multiple processes pointing at a single variable, one
thread can read and another thread can write. If they hit at the same
time, the reader thread is guaranteed to get one of the two values:
either before the change or after the change. What atomic does not
give you is any sort of guarantee about which of those values you
might get. Atomic is really commonly confused with being thread-safe,
and that is not correct. You need to guarantee your thread safety
other ways. However, atomic will guarantee that if you try to read,
you get back some kind of value.

nonatomic

On the flip side, non-atomic, as you can probably guess, just means,
“don’t do that atomic stuff.” What you lose is that guarantee that you
always get back something. If you try to read in the middle of a
write, you could get back garbage data. But, on the other hand, you go
a little bit faster. Because atomic properties have to do some magic
to guarantee that you will get back a value, they are a bit slower. If
it is a property that you are accessing a lot, you may want to drop
down to nonatomic to make sure that you are not incurring that speed
penalty.

Before discussing about the attributes of @property, you should know what is the use of @property.
@property offers a way to define the information that a class is intended to encapsulate. If you declare an object/variable using @property, then that object/variable will be accessible to other classes importing its class.
If you declare an object using @property in the header file, then you have to synthesize it using @synthesize in the implementation file.

atomic : It is the default behaviour. If an object is declared as atomic then it becomes thread-safe. Thread-safe means, at a time only one thread of a particular instance of that class can have the control over that object.

nonatomic: It is not thread-safe. You can use the nonatomic property attribute to specify that synthesized accessors simply set or return a value directly, with no guarantees about what happens if that same value is accessed simultaneously from different threads. For this reason, it’s faster to access a nonatomic property than an atomic one.
@property (nonatomic)NSString *name;

retain: is required when the attribute is a pointer to an object.The setter method will increase retain count of the object, so that it will occupy memory in autorelease pool.
@property (retain)NSString *name;

copy: If you use copy, you can't use retain. Using copy instance of the class will contain its own copy.
Even if a mutable string is set and subsequently changed, the instance captures whatever value it has at the time it is set. No setter and getter methods will be synthesized.

readonly: If you don't want to allow the property to be changed via setter method, you can declare the property readonly.
@property (readonly) NSString *name;

readwrite: is the default behaviour. You don't need to specify readwrite attribute explicitly.

@property (readwrite) NSString *name;

assign: will generate a setter which assigns the value to the instance variable directly, rather than copying or retaining it. This is best for primitive types like NSInteger and CGFloat, or objects you don't directly own, such as delegates.

unsafe_unretained: There are a few classes in Cocoa and Cocoa Touch that don’t yet support weak references, which means you can’t declare a weak property or weak local variable to keep track of them. These classes include NSTextView, NSFont and NSColorSpace,etc. If you need to use a weak reference to one of these classes, you must use an unsafe reference.
An unsafe reference is similar to a weak reference in that it doesn’t keep its related object alive, but it won’t be set to nil if the destination object is deallocated.

If you are using your property in multi-threaded code then you would be able to see the difference between nonatomic and atomic attributes. Nonatomic is faster than atomic and atomic is thread-safe, not nonatomic.

Vijayendra Tripathi has already given an example for a multi-threaded environment.

As many many many maaaaany others have said, atomic is NOT thread-safe! It's more resistant to thread problems, but not thread-safe. It just ensures you will get a whole value, a.k.a. a "correct" value (binary level), but by no means it will ensure that it is the current and "correct" value for your business logic (it could be a past value and invalid by your logic).
– Alejandro IvánFeb 15 '17 at 13:31

Before you begin: You must know that every object in memory needs to be deallocated from memory for a new write to happen. You can't just simply write on top of something as you do on paper. You must first erase (dealloc) it and then you can write onto it. If at the moment that the erase is done (or half done) and nothing has yet been wrote (or half wrote) and you try to read it could be very problematic! Atomic and nonatomic help you treat this problem in different ways.

First read this question and then read Bbum's answer. In addition then read my summary.

atomic will ALWAYS guarantee

If two different people want to read and write at the same time, your paper won't just burn! --> Your application will never crash, even in a race condition.

If one person is trying to write and has only wrote 4 of the 8 letters to written, then no can read in the middle, the reading can only be done when all 8 letters is written --> No read(get) will happen on 'a thread that is still writing', i.e. if there are 8 bytes to bytes to be written, and only 4 bytes are written——up to that moment, you are not allowed to read from it. But since I said it won't crash then it would read from the value of an autoreleased object.

If before writing you have erased that which was previously written on paper and then someone wants to read you can still read. How? You will be reading from something similar to Mac OS Trash bin ( as Trash bin is not still 100% erased...it's in a limbo) ---> If ThreadA is to read while ThreadB has already dealloced to write, you would could either get value from the final fully written value by ThreadB or get something from autorelease pool.

Retain counts are the way in which memory is managed in Objective-C.
When you create an object, it has a retain count of 1. When you send
an object a retain message, its retain count is incremented by 1. When
you send an object a release message, its retain count is decremented
by 1. When you send an object an autorelease message, its retain count
is decremented by 1 at some stage in the future. If an objectʼs retain
count is reduced to 0, it is deallocated.

Atomic doesn't guarantee thread safety, though its useful for achieving thread safety. Thread Safety is relative to how you write your code/ which thread queue you are reading/writing from. It only guarantees non-crashable multithreading.

Yes. Multithreading means: multiple threads can read a shared piece of data at the same time and we will not crash, yet it doesn't guarantee that you aren't reading from a non-autoreleased value. With thread safety, it's guaranteed that what you read is not auto-released.
The reason that we don't make everything atomic by default is, because there is a performance cost and for most things don't really need thread safety. A few parts of our code need it and for those few parts we need to write our code in a thread safe way using locks, mutex or synchronization.

nonatomic

Since there is no such thing like Mac OS Trash Bin, then nobody cares whether or not you always get a value (<-- This could potentially lead to a crash), nor anybody cares if someone tries to read halfway through you writing (although halfway writing in memory is very different from halfway writing on paper, on memory it could give you a crazy stupid value from before, while on paper you only see half of what's been wrote) --> Doesn't guarantee to not crash, because it doesn't use autorelease mechanism.

Doesn't guarantee full written values to be read!

Is faster than atomic

Overall they are different in 2 aspects:

Crashing or not because of having or not having autorelease pool.

Allowing to be read right in the middle of a 'not yet finished write or empty value' or not allowing and only allowing to read when the value is fully written.

The atomic property ensures to retain a fully initialised value irrespective of how many threads are doing getter & setter on it.

The nonatomic property specifies that synthesized accessors simply set or return a value directly, with no guarantees about what happens if that same value is accessed simultaneously from different threads.

If you are using atomic, it means the thread will be safe and read-only. If you are using nonatomic, it means the multiple threads access the variable and is thread unsafe, but it is executed fast, done a read and write operations; this is a dynamic type.

Atomic properties :- When a variable assigned with atomic property that means it has only one thread access and it will be thread safe and will be good in performance perspective, will have default behaviour.

Non Atomic Properties :- When a variable assigned with atomic property that means it has multi thread access and it will not be thread safe and will be slow in performance perspective, will have default behaviour and when two different threads want to access variable at same time it will give unexpected results.

Mutex lock, as per the name, locks the mutability of the object. So if the object is accessed by a class, no other class can access the same object.

In iOS, @sychronise also provides the mutex lock .Now it serves in FIFO mode and ensures the flow is not affected by two classes sharing the same instance. However, if the task is on main thread, avoid accessing object using atomic properties as it may hold your UI and degrade the performance.

Thank you for your interest in this question.
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